1. Field of the Invention
The present invention relates to the coalescing art, and specifically to an improved coalescing filter element which may be used in virtually any coalescing filter assembly. More particularly, the invention relates to a coalescing filter element used in separating liquid droplets from gases or other liquids, and having a conical configuration. The conical configuration allows for lower velocity of the primary phase fluid in the area between the outside surface of the coalescing element or elements and the filter assembly inner wall, thereby reducing the maximum droplet diameter which may be supported by or reentrained in the primary phase fluid. This allows for more efficient separation of the coalesced droplets from the primary phase fluid. It also allows for lower pressure drop through the coalescing filter elements.
2. Description of the Prior Art
The need to separate liquid droplets from gases or other liquids is long standing in the art. Common liquids found in air and gas streams include lube oils, water, salt water, acids, caustics, hydrocarbons, completion fluids, glycol and amine. The liquid normally is present in the form of tiny droplets, or aerosols. The size distribution of the aerosols is primarily dependent on the surface tension of the liquid contaminant and the process from which they are generated. As the surface tension is reduced, the size of the aerosol is reduced accordingly. This is because the intermolecular cohesive forces (the forces which attract the surface molecules of an aerosol inward in order to minimize surface area with respect to volume) are weaker.
It has been found that greater than 50% of all oil aerosols by weight are less than 1 micrometer in diameter. Due to their similar surface tensions, the same holds true for glycols, amines and hydrocarbons. Conventional filtration/separation equipment such as settling chambers, wire mesh (impingement) separators, centrifugal or vane (mechanical) separators and coarse glass or cellulose filters are only marginally efficient at 1 micrometer, and remove virtually none of the prevalent sub-micrometer aerosols and particles. In order to remove these problem-causing contaminants, high efficiency coalescing filters must be used.
All previous coalescing filters and coalescing elements of the type with which the present invention is concerned are configured in a tubular or cylindrical arrangement, and used to flow in to out or, from out to in. While it is advantageous to flow from out to in for many filter applications, there is also a definite advantage for flowing in to out for the coalescing of liquid droplets and aerosols from gases, or the coalescing of two immiscible liquid phases.
In these applications, it is common to use coalescing elements secured within a pressure-containing vessel or housing to form a coalescing filter assembly. The continuous phase gas or liquid contains dispersed liquid aerosol droplets, sometimes referred to as the discontinuous phase. The mixture enters the assembly through an inlet connection and then flows to the inside of the coalescing element. As the fluid flows through the filter media of the coalescing element, the liquid droplets come in contact with the fibers in the media and are removed from the fluid stream. Within the media, the droplets coalesce with other droplets and grow to emerge as large droplets on the downstream surface of the element which are capable of being gravitationally separated from the continuous phase fluid. If the density of the droplets is greater than that of the fluid, such as oil droplets in air, the droplets will settle gravitationally to the bottom of the filter assembly, countercurrent to the upward flow of air. If the density of the droplets is less than that of the fluid, such as oil droplets in water, the droplets will rise to the top of the assembly countercurrent to the downward flow of the water.
The droplet size, droplet density, fluid viscosity, and fluid density will determine how rapidly the droplet settles or rises in the filter assembly. It is advantageous in designing coalescing filter assemblies to try to maximize the flow rate of the fluid through the assembly while not reducing separation efficiencies in order to reduce the size of the housing required for a given flow rate, and thereby reduce the manufacturing costs.
However, the cylindrical coalescing elements of the prior art impose substantial limiting factors in designing filter housings. The cylindrical configuration of the coalescing elements provides a fixed annular space between the element and the housing wall. Therefore, assuming substantially even flow distribution across the surface of the coalescing element, the annular velocity increases linearly from the bottom to the top of the element.
With the cylindrical element design, the annular velocity will be different at all points along the axial length of the element. For example, in separating oil droplets from a gas, the gas will flow upward upon exiting the element, and the liquid droplets will settle downward. At the bottom of the element there would be no flow, so the annular velocity would be zero. At the top of the element, all of the gases would have exited the element and be flowing upward. The annular velocity would be 100% of the flow divided by the cross sectional open area (the area between the element and the vessel wall). Similarly, at a point in the middle of the element the annular velocity would be 50% of the total flow divided by the cross sectional open area. Great care must be taken not to exceed the annular velocity which will cause reentrainment of the droplets.
Furthermore, the pressure drop which results from the gas entering the open end of the element is a function of the inside diameter of the element. The inside diameter of cylindrical elements is limited by the diameter of the housing, the thickness of the wall of the element, and the size of the annular space. It is necessary to maintain sufficiently low annular velocities so as not to reentrain liquid droplets. The smaller the inside diameter is, the higher the pressure drop will be for a given flow rate.
After much study of the problem of how to reduce the annular velocity to prevent reentrainment of the liquid droplets, a substantially conically shaped coalescing filter was devised wherein the open area between the wall of the housing and the filter element increases in the direction of flow. The annular velocity can be expressed as V.sub.a =A/A.sub.x where Q is the flow and A is the open cross sectional area. It can be seen that if the area between the filter element and the housing increases as the flow increases, the annular velocity may be made to remain constant or, if desired, even to decrease.
Once the idea of a conical coalescing filter was developed, a search of the prior art in the United States Patent and Trademark Office was made to determine if this was new. The only patent located which discloses a conical coalescing element is U.S. Pat. No. 2,823,760 to S. K. Anderson entitled "Water Separator". Other patents were located during the search, but were not relevant. Upon close study of the Anderson "Water Separator", there was found to be a similarity in appearance only. Anderson deals with maintaining a constant pressure. It is not an in to out coalescer, and in fact is a centrifugal coalescer which flows out-to-in and relies on centrifugal force and subsequent steady flow rate to separate the coalesced droplets from the primary phase fluid. Thus even though Anderson thought of making a conically shaped coalescing filter cartridge, he did so for an entirely different purpose, and did not solve the problems in the coalescing filter art which Applicant addresses.